Tyre for vehicle wheels with improved bead structure

ABSTRACT

A tire for vehicle wheels includes a carcass, a tread strip, and a belt structure. The carcass has a central crown portion and two axially opposite sidewalls terminating in a pair of beads. Each bead includes at least one circumferentially unextendable annular reinforcing core having a series of spirals of metal wire radially superimposed and axially arranged alongside each other. The carcass has a reinforcing structure including at least one ply of rubberized fabric reinforced with metal cords lying in radial planes containing an axis of rotation of the tire. The reinforcing structure includes ends secured to the annular reinforcing cores and a neutral profile, lying in a radial cross-sectional plane, axially extending from bead to bead. The neutral profile intersects a cross section of a zone enclosing the annular reinforcing cores and has a continuous curvature devoid of inflection points along an extension between the beads.

[0001] The present invention relates to tires for motor vehicles andmore specifically to their beads, i.e. those annular carcass zones whichserve mainly to fix the tire onto the corresponding mounting rim.

[0002] More precisely the abovementioned invention relates to the systemfor joining together the annular reinforcing structures contained insidesaid beads, more commonly referred to by the name of “bead cores”, andthe ends of the carcass ply or plies.

[0003] A tire traditionally comprises a toroidally shaped carcass havinga central crown zone connected at its ends to a pair of axially oppositesidewalls which extend radially inwards and each terminate in a beadintended to fix the tire to a corresponding mounting rim. For thispurpose, the abovementioned reinforcing bead core is incorporated insidethe bead.

[0004] The crown of said carcass has, coaxially arranged on it, a treadstrip which is intended for the rolling contact of the tire on theground and is provided with a raised pattern defined by incisions andgrooves which are formed in the thickness of said strip and are designedto ensure the handling properties of the tire during use.

[0005] The reinforcing structure of the carcass comprises at least oneply of rubberized fabric consisting of a rubber sheet inside whichtextile or metal reinforcing cords are embedded, said cords beingarranged transversely with respect to the circumferential direction ofthe tire: in radial-carcass tires the direction of the abovementionedcords is perpendicular to said circumferential direction, i.e. to theequatorial plane of the tire.

[0006] Whatever the direction of the reinforcing cords, the ends of thecarcass ply are folded back axially from the inside towards the outsidearound the abovementioned bead cores.

[0007] Owing to this geometrical arrangement, the profile of the carcassply along the cross section of the tire, and therefore also the profileof the corresponding reinforcing cord in radial-type carcasses, has aninflection point at the axially internal and radially external edge ofthe bead core, where the curvature of the abovementioned profile isreversed, changing from concave, along the sidewall, to convex, aroundthe bead core.

[0008] In the continuation of the present description the abovementionedinflection point will be called the “K point”.

[0009] When the carcass is of the radial type, it also comprises a beltstructure arranged on the crown of the carcass, being located betweencarcass and tread strip and extending from one side to the other of thetire, i.e. is substantially as wide as the tread strip.

[0010] The abovementioned structure traditionally comprises one or morepairs of strips of rubberized fabric provided with reinforcing cordswhich are parallel to one another in each strip and intersecting thoseof the adjacent strips, being inclined preferably in a symmetricalmanner with respect to the equatorial plane of the tire.

[0011] The tire mounting rims have, at their axial ends, two more orless conical coaxial surfaces which form the support base for the tirebeads. The axially external edge of said bases terminates in a flangewhich extends radially outwards and acts as a support for the axiallyexternal surface of the bead and against which the abovementioned beadis kept pressed by the inflation pressure of the tire.

[0012] The forced engagement of the tire bead inside its seat is ensuredby the conicity of the support base, which is open outwards, togetherwith the metal reinforcing bead core, which is unextendablecircumferentially and therefore has a constant diameter and is containedin the bead of the tire: this forced engagement, which is produced bythe axial thrust exerted on the side of the beads, axially from theinside towards the outside, by the inflation pressure of the tire,ensures the stability of the tire bead on the rim during use and, in thecase of tubeless tires, also airtightness between the tire and rim so asto prevent gradual deflation of the tire.

[0013] In order to guarantee fully this airtightness, the tubeless tireis mounted on so-called drop center rims, i.e. rims made as one piecewith bead support bases which have a high degree of conicity, i.e. areinclined at 15° relative to the axis of rotation of the rim.

[0014] This particular inclination of the bead bases means that the tiremust have a particular profile of the carcass ply, distinguished by atangent to said profile at the K point which is very inclined withrespect to the axis of rotation of the rim.

[0015] All the above having been stated, the tires to which theinvention preferably relates are tubeless tires for road transportationwhich have a metal, radial, single-ply carcass and which are used withworking pressures of the order of eight bar.

[0016] In these tires the bead zone is a structurally very critical areawhich very often results in disuse of the tires owing to structuralweakness in this zone well before the tread has become completely worn.

[0017] Many attempts have already been tried in the art in order toimprove the situation, without, however, succeeding in solving theproblem: these include, in particular, modification of the cross sectionof the bead core and that of the steel wire from which the bead core ismade, if necessary also in combination with one another, in order toprovide the bead core with greater torsional rigidity, as well asmodification of the profile of the carcass ply and the attempt tooptimize the value of the radial height of the folded-back portions ofthe carcass ply.

[0018] All these attempts have been unable to provide a completelysatisfactory solution and have often given rise to problems which areeven more serious than those which they were intended to solve.

[0019] In particular, the increase in torsional rigidity of the beadcore has resulted in the simultaneous increase in its flexural rigidity,making mounting and removal of the tire onto/from the rim extremelydifficult: in turn, modification of the profile of the carcass ply hashad a negative effect on many behavioural characteristics of the tireduring use.

[0020] According to Applicants' perceiving the high inflation pressurenot only induces in each cord of the carcass a pulling force which istransmitted onto the bead core along two components which areperpendicular to one another, i.e. an axial component and radialcomponent, and which are opposed by the circumferentialnon-deformability of the bead core, but also applies to the bead core atwisting torque due to the distance of the K point from the center ofmass of the bead core.

[0021] This twisting torque, during use of the tire, varies with eachrotational cycle of the tire, producing cyclical microscopic movementsin the whole structure of the bead and in particular microscopicrotations of the bead about its axially external edge and the edge ofthe flange, which result in the more or less rapid end of the life ofthe tire.

[0022] It has been found that the problem may be advantageously overcomeby causing the neutral profile of the carcass plies to pass inside thebead core, preferably through its center of mass, thus eliminating theinflection point.

[0023] The present invention relates to a new bead structure, inparticular for giant-size tubeless tires, which offers all theadvantages of flexibility, elastic deformability and mechanical strengthof the known beads, but which is devoid of the disadvantages illustratedhitherto.

[0024] The present patent also relates to a new bead structure whichprovides the tire with a greater endurance.

[0025] Moreover the present patent relates to a new bead structure whichprovides the tire with a smaller rolling resistance.

[0026] A different object of the present patent is a new bead structurewhich provides the tire with a greater load capacity compared to knowntires which have the same dimensions.

[0027] A further different object of the present patent is a new beadstructure which simplifies the tire manufacturing process and allows theuse of less costly machines.

[0028] Another different object of the present patent is a method forincreasing the load capacity of a tire for vehicle wheels compared toknown tires of the same size.

[0029] Another different object of the present patent is a method foreliminating the twisting torque, in the plane of the radial crosssection, applied to the annular bead reinforcing cores in a tire forvehicle wheels.

[0030] According to a first aspect thereof, the invention thereforerelates to a tire for vehicle wheels, comprising a torus-shaped carcasswhich has a central crown portion and two axially opposite sidewallsterminating in a pair of beads for fixing the tire onto a correspondingmounting rim, each bead comprising at least one circumferentiallyunextendable annular reinforcing core, a tread strip which is located onthe crown and coaxially extends around said carcass and is provided witha raised pattern for rolling contact with the road, and a belt structurecoaxially arranged between said carcass and said tread strip, saidcarcass being provided with a reinforcing structure essentiallyconsisting of at least one ply of rubberized fabric which is reinforcedwith metal cords lying in radial planes containing the axis of rotationof the tire, said reinforcing structure having its ends secured to saidannular reinforcing cores, and a neutral profile, in a radialcross-sectional plane, axially extending from bead to bead,characterized in that said neutral profile intersects the cross sectionof the zone which encloses said annular reinforcing cores and in thatthe ends of said reinforcing structure extend radially inwards notbeyond the radially innermost profile of said annular reinforcing cores.

[0031] According to a second aspect thereof, the invention relates to amethod for increasing the load capacity of a tire for vehicle wheels,provided with a conventional carcass structure as described above, saidmethod comprising the steps of limiting the radial extension inwards ofthe ends of the carcass structure so that they do not extend beyond theradially innermost profile of the annular bead-reinforcing cores, and ofcausing the neutral profile of the carcass plies to intersect the crosssection of the zone which encloses said annular reinforcing cores.

[0032] According to a further aspect thereof, the invention relates to amethod for eliminating the twisting torque, in the plane of the radialcross section, which is applied to the annular bead-reinforcing cores ina tire for vehicle wheels as described above, said method comprising thesteps of eliminating the inflection points along the extension of theneutral profile of the carcass reinforcing structure, and of causingsaid neutral profile to intersect the cross section of the zone whichencloses said annular reinforcing cores.

[0033] In any case, the present invention will be better understood nowwith the aid of the description which follows and the accompanyingdrawings, both of which are provided solely by way of a non-limitingexample and in which:

[0034]FIG. 1 shows, in partial cross section and in schematic form, thepath of the neutral profile of the carcass plies of the tire accordingto the invention, compared with that of the known art;

[0035]FIG. 2 shows, in partial cross section and in schematic form, afirst embodiment of a carcass structure of the tire according to theinvention, still compared with that of the known art;

[0036]FIG. 3 shows, in partial cross section, a second preferredembodiment of the carcass structure of the tire according to FIG. 1;

[0037]FIG. 4 shows the state of forces acting on the tire bead, viewedin partial cross section, during use;

[0038]FIG. 5 shows the mean progression, pointwise, of the pressureexerted by the tire bead on the bead seat of the rim;

[0039]FIG. 6 shows a bead model according to the invention to which allthe unitary forces which are transmitted onto the tire bead during useof the tire have been applied.

[0040]FIG. 1 therefore shows, in partial cross section and in schematicform, a tire according to the invention with specific reference to thepaths of its contour profiles and the neutral profile of the carcassplies, compared to the corresponding profiles of the tire according tothe known art.

[0041] The tire comprises a torus-shaped carcass which has a centralcrown portion C and two axially opposite sidewalls A, each terminatingin a bead B for fixing the tire onto a corresponding mounting rim. Thebead incorporates internally at least one circumferentially unextendableannular reinforcing core, which is usually referred to as a “bead core”and in respect of which the present figure shows only the center of massG and the profile S of the zone containing its cross section.

[0042] On the crown of the carcass, coaxially extending around it, thereis arranged a tread strip M which is provided with a raised pattern forrolling contact with the road, not illustrated in that it is of littlerelevance for the purposes of the invention, and which determines thehandling of the tire during use in particular as regards the tractionalcapacity, the is road-holding performance and the resistance toaquaplaning.

[0043] The carcass is provided with a reinforcing structure comprisingat least one ply of rubberized fabric reinforced with textile and/ormetal cords which form the reinforcing element extending from bead tobead and secured to the abovementioned bead cores.

[0044] Only the profile N of this structure is shown, as will be seenbelow.

[0045] The profile E indicates the external contour of the tire, whichis common both to the tire of the invention and to that which is known,in order to illustrate better the comparison between the tire accordingto the invention and the known tire using equivalent tires with the samemaximum overall dimensions and the same size.

[0046] The profiles I₁, and I₂ indicate the internal tire profile forthe tire according to the invention and the known tire, respectively,which are spaced from one another in the radially internal portion ofthe sidewall and in the bead.

[0047] The profiles N₁ and N₂ indicate the profile of the carcass ply orplies, for the tire according to the invention and the known tire,respectively, which are also spaced from one another in the radiallyinternal portion of the sidewall and in the bead.

[0048] As regards the tire of the known type, the profile N₂ has alongits extension, in the region of the bead, an inflection point K where,owing to winding of the carcass plies around the bead core, thecurvature of the profile changes direction, becoming outwardly concaveinstead of inwardly concave. More particularly, the K point is locatedat the axially internal end of the bead core and sometimes morespecifically substantially at the axially internal and radially externaledge of the bead core. The tangent to the profile N₂ at the point K hasan angle of inclination “w” which is relatively small with respect tothe axis of is rotation of the tire, being by way of a rough guidebetween 20° and 35°.

[0049] The profile N₁ of the tire according to the invention insteadpasses inside the zone which contains the cross section of the beadcore, or the group of bead cores, forming the bead reinforcement, andpreferably passes through the center of mass G of the bead core or ofthe abovementioned group of reinforcing bead cores; for the sake ofconvenience, the reference zone is taken as being that delimited by acurved fine S which passes through the vertices of the bead core orcores and which minimizes the value of the area of the zone, i.e. thecircumscribed area.

[0050]FIG. 1 clearly shows how the abovementioned profile N₁ is devoidof the K point, does not have reversals in curvature along its extensionand has an inclination “y” with respect to the axis of rotation of thetire, which is defined by the inclination of the tangent to the profileN₁ passing through the center of mass G, which is markedly greater thanthat of the tangent passing though the abovementioned K point in theknown tires.

[0051] In the following of the present description, reference will bemade to the neutral profile of the carcass plies: in fact, this profilecoincides with the profile of said plies when the latter are in mutualcontact, but diverges therefrom when the plies move away from oneanother. In this case the neutral profile corresponds to the profile ofthe neutral axis of the assembly externally delimited by two or morecarcass plies and incorporating internally further filling orreinforcing elements such as compounds and/or fibers, yarns, threads orcords made of textiles or metal or other suitable material.

[0052] It is worth recalling here that the profile of the carcass pliesis commonly referred to in technical literature by the term “ply line”;the portion of said profile, which is deformed upon variation in theload and the inflation pressure of the tire providing the tire with loadcapacity and vertical resilience, is commonly indicated in technicalliterature by the term “ρy” and corresponds substantially to the portionof the ply line radially outside the K point. The “ρy” is also referredto here as the “active profile”.

[0053] It is immediately apparent from FIG. 1 how the neutral profile ofthe carcass plies according to the invention allow a substantialreduction in the thickness of the bead and a widening of the activeprofile as far as the center of mass of the bead core. This reduction inthickness, i.e. the elimination of the hatched bead portion in thefigure not only increases the volume of air contained in the tire by aquantity approximately between 2% and 5%, but concentrates this increasein the radially innermost portion of the tire. The result is thedisplacement of the volumetric center line of the tire towards the axisof rotation of the rim with a corresponding increase in the loadcapacity of the tire itself, conditions being equal with the equivalenttire of the art.

[0054] The streamlining of the bead produces, per se, a reduction in theweight of the tire and its operating temperature owing to the smallerquantity of material subject to the cyclical deformations associatedwith the rolling movement of the tire.

[0055]FIG. 2, shows, still in partial cross section and in schematicform, a first embodiment of a carcass structure of the tire according tothe invention, still compared with the equivalent carcass structure(shown in broken lines) of the known art.

[0056] In addition to that already explained and illustrated withreference to FIG. 1, the tire according to the invention comprises acarcass ply 1, reinforced with metal cords arranged radially, i.e. lyingsubstantially in the plane of the cross section of the tire. The carcassply 1 may consist of a single sheet of rubberized fabric or a group ofseveral sheets or a series of bands suitably arranged as describedbelow.

[0057] The end of the abovementioned ply is inserted between two beadcores 2 a and 2 b which together form the annular reinforcing core ofthe tire bead. Preferably the end of the ply does not project from thebead core radially inwards, i.e. does not extend radially inwards beyondthe radially innermost profile “p” of the bead core or group of beadcores, and therefore cannot even be folded around a bead core, contraryto the known structures where the end 3 rotating axially from the insidetowards the outside encompasses the bead core 2 and rises up radiallyoutwards along the side of the carcass ply, causing a structuraldiscontinuity in the bead at the height of the lip 4 of theabovementioned fold.

[0058] The effect of this discontinuity has to be usually offset bymeans of the insertion of a textile or metal side-piece 5 radiallyextending on both sides of said lip.

[0059] The elimination of the fold of the carcass ply around the beadcore, both from the inside towards the outside and vice versa,simplifies the tire manufacturing process and eliminates an importantsource of negative consequences.

[0060] In one particular embodiment, the carcass according to theinvention also comprises a belt structure 6 which is coaxially arrangedon the crown of the carcass and located between the abovementionedcarcass and the tread strip M.

[0061]FIG. 3 shows a preferred embodiment of the tire according to theinvention, which has been constructed using the technology described inthe co-pending European patent application filed by the same Applicantsunder No. EP 97-830731.2.

[0062] The internal components of the tire are placed on a torus-shapedmale drum 20, the external profile of which coincides with that of theradially internal surface of the unprocessed tire, starting with theso-called liner 7, i.e. a sheet of rubber which, in the vulcanized tire,forms the internal airtight surface of the tire.

[0063] The carcass ply is then constructed by placing on the liner, incircumferential succession, a plurality of bands 8, i.e. strips ofrubberized fabric each containing a certain number of steel cords, saidbands being arranged spaced from one another with a widthwise intervalpreferably equal to the width of the band, with the cords arrangedradially, i.e. at 90° with respect to the circumferential direction ofthe male drum. The bands are fixed to the liner over the whole of theirlongitudinal extension, extending from bead to bead along the externalsurface of the male drum.

[0064] Preferably the bands have a width of between 3 and 15 mm (evenmore preferably between 5 and 10 mm), a thickness of between 0.5 and 2.5mm, and contain a number of cords ranging between 2 and 15 (even morepreferably between 3 and 10), with a density of preferably between 4 and8 cords per centimeter, measured on the carcass ply, in thecircumferential direction, in the vicinity of the equatorial plane ofthe tire 1.

[0065] Preferably the cords are arranged in the bands 8 at a mutualinteraxial distance of not less than 1.5 times the diameter of thecords, so as to allow adequate rubberization between the adjacent cords.

[0066] However, if required, the cords may advantageously be arranged inthe bands also with a greater density, so as to provide the carcass plywith particular properties of compactness and homogeneity.

[0067] The cords are those which are usually used in the preparation oftire carcasses, preferably with a diameter of between 0.3 mm and 2.1 mm.For the preparation of a prototype tire of size 315/80 R 22.5″ theApplicants have found it convenient to use metal cords with thecomposition 3+9+15×0.22.

[0068] A reinforcing bead core 9 is now applied onto this first seriesof bands, in the region of the bead, together with, in a positionradially outside said bead core , a triangular-shaped filling body 10which is tapered towards the opposite end of the bead core, radiallyextending outwards over a radial eight “h” which is preferably between10% and 50% of the height of the section H of the tire.

[0069] The bead core 9 may be constructed separately, on a suitablepreparation drum, and then assembled on the carcass under construction,or may be constructed directly on the male drum, above theabovementioned bands, in the region of the bead.

[0070] Preferably, the bead core comprises a metal annular core which iscircumferentially unextendable and consists of a series of spirals ofmetal wire which are radially superimposed and axially arrangedalongside each other.

[0071] The series of spirals may be formed by winding onto a suitabledrum, in the form of several spirals radially superimposed on oneanother, a wire or a strip of metal wires arranged alongside each other,or by arranging alongside each other, in succession, a plurality ofmetal spirals which are made by winding about itself, in radiallysuperimposed spirals, a single metal wire, or also by superimposingradially with respect to one another a plurality of metal layers whichare formed by winding, in spirals axially arranged alongside each other,a single metal wire.

[0072] It should be noted that, owing to the fact that the neutralprofile of the carcass plies passes inside the zone which encloses thecross section of the bead core, and more specifically through the centerof mass of the bead core itself, the latter is no longer subject to atwisting torque, so that it merely has to withstand tractional forcesapplied to its cross section.

[0073] As a result, the bead core is no longer required to have aspecific torsional strength, and therefore characteristics such asnon-deformability of its cross section or inclination of the radiallyinternal surface of the bead core at an angle substantially equal tothat of the bead seat on the rim become less important.

[0074] This fact results in a considerable freedom of choice in the formof the cross section of the bead core and the wire forming it.

[0075] As regards the cross section of the bead core 9, in theembodiment described, the latter is preferably formed with an irregulartrapezoidal shape, comprising two bases, respectively consisting of theradially internal surface 9 a and the radially external surface 9 b, andtwo inclined sides, respectively consisting of the axially internalsurface 9 c and the axially external surface 9 d. The base 9 a isinclined with respect to the axis of rotation of the tire at an anglesubstantially equal to that of the bead base on the rim, i.e. of theorder of 15°, but may also have a smaller inclination, preferably ofbetween 0° and 12°, so as to favour the forced engagement of the beadonto the corresponding seat. According to another preferred aspect, theangle of inclination of the axially external and axially internal sidesof said trapezium with respect to the axis of rotation of the tire isrespectively greater than and less than the angle of inclination of theneutral profile of the carcass plies in is the region of said sides.

[0076] Of particular importance are the linear dimensions of the beadcore 9, i.e. the extension (z) in the longitudinal direction (inclinedsides) and axial thickness (v) (radial bases) in the transversedirection, shown in FIG. 3; preferably the dimension z is between 10 and25 mm, while the dimension v is between 7 and 20 mm.

[0077] The material of the bead core may be any textile or metallicmaterial, or any material of another kind, which has suitable mechanicalstrength characteristics; preferably this material is normal steel wireor high carbon-content (high tensile) steel wire, commonly used in tiretechnology, or a metal cord with a corresponding load capacity andstrength.

[0078] In the case where metal wire is used, its cross section ispreferably circular, with a diameter of between 0.3 and 2.1 mm;alternatively a flat metal strip may be used, preferably with asubstantially rectangular cross section, with sides having dimensionsranging between 1.5 and 2.5 mm, preferably with rounded edges.

[0079] In a different embodiment, the flat strip may have asubstantially hexagonal cross section for example as described in U.S.Pat. No. 5,007,471 in the name of the same Applicants.

[0080] In the case where a metal cord is used, the Applicants have foundit advantageous to use the cord 7×4×0.28, i.e. a cord consisting ofseven strands of four wires each, each wire having a diameter of 0.28mm.

[0081] Preferably the metal elements (wires or strips) which form thespirals of the bead core 9 are bare, i.e. not lined with rubber; in thiscase, each metal element retains the possibility of effecting smallcircumferential displacements with respect to the adjacent metallicelements, thus maximizing the flexibility of the bead core, also in thecase of axial interconnection between the sections of the adjacentstrips, and hence the possibility of ovalization of the bead wire duringmounting of the tire onto the rim.

[0082] A new series of bands 8 b are now placed in the spaces leftbetween the bands 8 a arranged in position previously: the ends of thesebands are superimposed in a position axially outside the bead corealready assembled on the male drum, such that the bead core is arrangedbetween two layers of carcass cords. In particular, in the region of thebeads, the bands are positioned alternately against the axially oppositesides of the bead core: the bands of the first series are axiallyinternal, the bands of the second series are axially external and notlocated opposite those of the first series. In other words, the carcassthus formed behaves in the manner of a radial single-ply carcass on thecrown and along the radially external center line of the side wall, andas a dual-ply carcass along the radially internal center line of thesidewall and in the region of the beads. In this way the carcassaccording to the invention embraces all the advantages of single-plycarcasses in terms of lightness, comfort and flexibility and all theadvantages of dual-ply carcasses in terms of structural strength,vertical and torsional rigidity of the beads, and stability in thepresence of lateral displacement forces.

[0083] In a first embodiment, all the bands are identical to oneanother, i.e. have the same number and the same type of reinforcingcords. However, it may be convenient to vary the characteristics of thebands secured in a position axially inside the bead core from thosesecured in a position axially outside the bead core: in a first variant,the so-called axially internal bands have a different width, preferablygreater than the so-called axially external bands such that, for thesame density of the cords in the bands, the cords axially inside thebead core are greater in number than the axially external cords, whilemaintaining a uniform density within the carcass ply.

[0084] In a second variation of embodiment, the cords axially inside thebead core are instead of a different type, i.e. are preferably stronger,compared to the axially external cords; it will be clear that in thiscase the carcass ply will have, on its crown, a periodic circumferentialvariation in uniformity, which if necessary can be restored by modifyingthe density of the cords in the various bands.

[0085] In yet another alternative variation of embodiment, the bead ofthe tire according to the invention has a second reinforcing bead core11, which is preferably arranged axially outside the carcass bands in anaxially external position. Preferably this second bead core comprises acircumferentially unextendable annular insert which is formedsubstantially in the manner of a circular rim concentric with the axisof rotation of the tire and located in an axially external positionagainst the end portions of the abovementioned bands.

[0086] This annular insert is composed of at least one elongated metalelement wound in several substantially concentric spirals. The spiralsmay be defined by a continuous spiral or by concentric rings formed byrespective elongated metal elements.

[0087] In a different embodiment, the annular insert has the same form,possibly with different dimensions, as the trapezoidal bead core in anaxially internal position.

[0088] The neutral profile of the carcass ply, which coincides with thatof the ply in the portion where the bands follow the same trajectory,moves away from the bands when these separate from each other, at theradially external end of the filling body, and passes between the twoseries of bands, inside the area which encloses the cross section of thebead core or of the two bead cores described above, and more preferablypasses through the center of mass of the bead core or of the group ofbead cores: in this way the active profile of the carcass continuesradially inwards as far as a point in the immediate vicinity of the beadcore.

[0089] The carcass is then completed with the arrangement of the beltstructure 6, the sidewalls 12 and the tread strip M, and is then moldedand vulcanized: if necessary owing to particular requirements, thestructure of the bead may comprise other reinforcing elements such astextile fabric and/or metal strips in an axially internal and/orexternal position and/or fillets of blend with particularcharacteristics, such as for example, an anti-abrasive fillet 13 in anaxially external position which protects the tire bead from rubbingagainst the rim shoulder.

[0090] For example, it may comprise a reinforcing edge 14 consisting ofa strip of rubberized fabric reinforced with cords made of any material,preferably metallic material. The cords are arranged in the rubberizedfabric at an angle with respect to the radial direction of the tire ofbetween 0° and 60°, and more preferably between 15° and 25°. The heighth₂ of the reinforcing edge is preferably between 15% and 35% of theheight of the section H of the tire.

[0091] In another alternative embodiment of the present invention, thebands of the first series are positioned alongside each other or arespaced with an interval equal to the smallest width of the width of thebands of the second series. The positioning of these second bands isperformed by superimposing them entirely on top of the underlying bands,thus resulting in a dual carcass ply also on the crown and in the radialexternal portion of the sidewalls. In this case it is possible to obtaingreater strength of the carcass against the increase in weight of thetire. With this solution it is also possible to arrange the radiallysuperimposed bands with the reinforcing cords of the first seriesintersecting those of the second series and if necessary at differentangles of inclination to one another with respect to the circumferentialdirection.

[0092] In this embodiment of the present invention it is advantageous tovary the thickness, measured circumferentially on the carcass ply in thevicinity of the equatorial plane of the tire, of the cords in the firstseries and the second series of bands and in particular to use a greaterthickness of cords for the bands 8 a of the first series, i.e. thosewhich are radially innermost, with respect to those 8 b of the secondseries. For example, it is possible to use a thickness of 8 cords percentimeter for the first series and 4 cords per centimeter for thesecond series. In order to verify the technical validity of the proposedsolution, the Applicants have constructed a model of the bead accordingto the invention, to which all the unitary forces which are transmittedonto the tire bead during use of the tire were applied.

[0093] With reference to FIG. 4 these main forces are:

[0094] the force P due to the inflation pressure

[0095] the force Q corresponding to the load

[0096] the force T due to the pulling force exerted by the carcass plyand more particularly by the reinforcing cords of the carcass ply, owingto the inflation pressure.

[0097] These forces, which are indicated overall by P1, are transmittedonto the shoulder of the rim (respectively at the points B1 to B6) andvary cyclically in intensity and in direction during rolling of the tireunder load. The diagram in FIG. 5 shows the mean progression pointwise(from B1 to B6) of the pressure P1 exerted by the tire bead on the rimshoulder.

[0098] These mean forces, which have been measured and calculated forthe prototype tire mentioned above, with the size 315/80 R 22.5″, wereapplied to the model illustrated in FIG. 6, said model having beenconstructed using the same materials and with the same geometrical formas the abovementioned tire.

[0099] The model was formed with a band 8 of rubberized fabric (length12 cm, width 1 cm, thickness 2.6 mm) made of the same fabric as thecarcass ply and comprising four steel reinforcing cords, with thecomposition 3+9+15×0.22, which are normally used in the carcass of thetires of the abovementioned size.

[0100] One end of the abovementioned band was enclosed between tworetaining elements 15 and 16 each consisting of two strips of rubberizedfabric, with a thickness of 2.3 mm and height of 20 mm, reinforced witheight metal cords with the composition 7×4×0.28 which are arrangedperpendicularly with respect to the cords of the carcass strip, so as tosimulate the bead core as shown in FIG. 2.

[0101] The model was vulcanized at the temperature of 190° C. for aduration of 30′ and at a pressure of 30 bar.

[0102] By means of a vice the two retaining elements were pressedtogether with a pressure P of 60 bar, equal to the mean value of thepressure with which the tire bead is pressed against the rim shoulder,while the opposite end of the band was subjected to a tractional force Tby means of a clamp to which a tractional load was applied, said loadincreasing until it caused laceration of the compound between the cordsof the band and those of the retaining elements.

[0103] The force F which caused the abovementioned laceration wasgreater than 400 kg and therefore greater than 100 kg per cord.

[0104] Since the pulling force T applied to each carcass cord duringrolling of the tire is about 20 kg, it follows that the safetycoefficient of the bead structure according to the invention issubstantially equal to 5, i.e. completely in line with that normallyused during the correct design of the tires according to the state ofthe art.

[0105] The tire according to the invention achieves many importantadvantages, all of which substantially result from the elimination ofthe twisting torque acting on the rim of the tires of the known art.

[0106] Firstly, the microscopic movements of the tire bead on the beadbase and around the rim shoulder are drastically reduced in magnitude,if not completely eliminated, with a greater working life of the tireowing to the smaller degree of fatigue of the critical zone of the beadduring rotation under load.

[0107] A further positive effect in connection with the greater workinglife of the tire is obtained as a result of the elimination of thestructural discontinuity caused in the known tires by the presence ofthe fold of the carcass ply, which is the cause of failures inparticular on the edge of the abovementioned fold.

[0108] It is worth remembering here that the tire for heavy load, whenworn, must be able to be covered with a new tread strip, at least once,but preferably several times, in view of the impact of this possibilityon the operating costs of the vehicle; however this operation ispossible only on carcasses which are integral so that the carcasses sentfor retreading are examined beforehand and suitably selected. Theimportance of the invention is clear considering that most of thecarcasses discarded prior to retreading are eliminated owing to beaddefects.

[0109] Owing to its reduced geometrical dimensions, the bead core or thepair of bead cores according to the invention form an annularreinforcing core which is very flexible and therefore capable of beingeasily deformed, assuming in particular the elliptical (oval)configuration necessary for allowing the bead to pass over the rimshoulder during mounting of the tire on the rim and, vice versa, removalof the tire from the rim, said operation being particularly complex inthe case of the giant-sized tires used for road transportation.

[0110] In addition to the above, the tire according to the inventionreduces the fuel consumption owing to the smaller rolling resistanceachieved as a result of the lower weight of the tire, since theelimination of a twisting moment applied to the bead allows both thebead core and the overall structure of the bead to be made lighter, withthe associated reduction in the hysteretic masses and deformations ofthe materials which form the abovementioned structure.

[0111] The load capacity of the vehicle is also increased since theactive profile of the carcass ply is radially extended inwards as far asthe center of the bead core with consequent expansion of the toroidalcavity, for the same total volume of the tire, and lowering of themeridian plane of the toroid towards the axis of rotation of the tire.It should be remembered here that the load-bearing capacity of the tireis directly proportional to the volume of the toroidal cavity comprisedbetween the radially external surface of the rim and the abovementionedvolumetric meridian plane.

[0112] In connection with that, it is now clear that the invention alsocomprises a method for increasing the load capacity of traditional tiresfor vehicle wheels, which are provided with a carcass structure such asthat described above.

[0113] In accordance with this method, the known process ofmanufacturing the tire is modified so as to limit the radial extensioninwards of the ends of the carcass reinforcing structure, so that theydo not exceed the radially innermost profile of the bead cores, thusmaking folding back of the carcass ply around the bead cores impossible,and the carcass plies are arranged in an axial position with respect tothe bead core such that the neutral profile of the abovementioned pliesis made to intersect the cross section of the zone which encloses thebead reinforcing cores.

[0114] The simplification of the production process and the associatedmanufacturing machinery resulting from elimination of folding of thecarcass ply around the bead core is a further not insignificantadvantage offered by the invention.

[0115] Finally, the invention also offers a method for eliminating thetwisting torque applied, in the plane of the radial cross section, tothe bead cores reinforcing the bead in tires for vehicle wheelsdescribed above, thus resulting in said advantages being achieved, inparticular as regards the lower weight of the tire and smaller rollingresistance.

[0116] The abovementioned method differs from the known tiremanufacturing methods since it comprises the steps of eliminating theinflection points along the extension of the neutral profile of thecarcass reinforcing structure and causing the abovementioned neutralprofile to intersect the cross section of the zone which encloses theannular bead reinforcing cores, i.e. the already much-cited bead cores.

[0117] It will also be clear that the person skilled in the art, havingnow understood the general features of the invention, will be able tomake all those structural modifications and variations to criticalparameters which, although not expressly described here, maynevertheless be easily deduced from the present description which isprovided merely by way of a non-limiting example, without therebydeparting from the protective scope of the present invention as claimedhereinbelow.

1. Tire for vehicle wheels, comprising a torus-shaped carcass which hasa central crown portion and two axially opposite sidewalls terminatingin a pair of beads for fixing the tire onto a corresponding mountingrim, each bead comprising at least one circumferentially unextendableannular reinforcing core, a tread strip which is located on the crownand coaxially extends around said carcass and is provided with a raisedpattern for rolling contact with the road, and a belt structurecoaxially arranged between said carcass and said tread strip, saidcarcass being provided with a reinforcing structure essentiallyconsisting of at least one ply of rubberized fabric which is reinforcedwith metal cords lying in radial planes containing the axis of rotationof the tire, said reinforcing structure having its ends secured to saidannular reinforcing cores, and a neutral profile, lying in a radialcross-sectional plane, axially extending from bead to bead,characterized in that said neutral profile intersects the cross sectionof the zone which encloses said annular reinforcing cores and in thatthe ends of said reinforcing structure extend radially inwards notbeyond the radially innermost profile of said annular reinforcing cores.2. Tire according to claim 1, characterized in that said neutral profilealong its extension between said beads has a continuous curvature devoidof inflection points.
 3. Tire according to claim 1, characterized inthat said annular reinforcing core comprises a first axially innermostbead core and a second axially outermost bead core, one end of saidcarcass ply being inserted between said first and second bead cores. 4.Tire according to claim 1, characterized in that said carcass plycomprises a plurality of rubberized-fabric bands which are alternatelyarranged, in at least one of said beads, on axially opposite sides ofsaid annular reinforcing core.
 5. Tire according to claim 4,characterized in that said carcass ply comprises two series of bandswhich are radially superimposed at least on the crown of said tire. 6.Tire according to claim 1, characterized in that said annularreinforcing core, in the plane of its cross section, is formed with anirregular trapezoidal shape which comprises two bases, i.e. a radiallyinternal base 9 a and a radially external base 9 b, and two inclinedsides, i.e. an axially internal side 9 c and an axially external side 9d.
 7. Tire according to claim 6, characterized in that the angle ofinclination, with respect to the axis of rotation of the tire, of theaxially external and axially internal sides of said trapezium isrespectively greater than and smaller than the angle of inclination ofsaid neutral profile of the carcass plies in the region of said sides.8. Tire according to claim 6, characterized in that the longitudinaldimension (z) of the inclined sides of the bead core (9) is between 10and 25 mm and the transverse dimension (v) of the bases of said beadcore is between 7 and 20 mm.
 9. Tire according to claim 1, characterizedin that said annular reinforcing core comprises a series of spirals ofmetal wire which are radially superimposed and axially arrangedalongside each other.
 10. Tire according to claim 9, characterized inthat the material of said spirals of metal wire is steel with a highcarbon content.
 11. Tire according to claim 9, characterized in that thecross section of said metal wire is hexagonal.
 12. Tire according toclaim 1, characterized in that said annular reinforcing core comprises aseries of radially superimposed spirals of flat metal strip.
 13. Tireaccording to claim 1, characterized in that said annular reinforcingcores are not subject to a twisting torque owing to the inflationpressure of the tire.
 14. Method for increasing the load capacity of atire for vehicle wheels, comprising a torus-shaped carcass which has acentral crown portion and two axially opposite sidewalls terminating ina pair of beads for fixing the tire onto a corresponding mounting rim,each bead comprising at least one circumferentially unextendable annularreinforcing core, said carcass being provided with a reinforcingstructure essentially consisting of at least one ply of rubberizedfabric which is reinforced with metal cords lying in radial planescontaining the axis of rotation of the tire, said reinforcing structurehaving its ends secured to said annular reinforcing cores, and a neutralprofile, lying in a radial cross-sectional plane, axially extending frombead to bead, said method comprising the steps of: limiting the radialextension inwards of the ends of said reinforcing structure so that theydo not extend beyond the radially innermost profile of said annularreinforcing cores; causing said neutral profile to intersect the crosssection of the zone which encloses said annular reinforcing cores. 15.Method for eliminating the twisting torque, in the plane of the radialcross section, which is applied to the annular bead-reinforcing cores ina tire for vehicle wheels comprising a torus-shaped carcass which isprovided with a reinforcing structure essentially consisting of at leastone ply of rubberized fabric which is reinforced with metal cords lyingin radial planes containing the axis of rotation of the tire, saidreinforcing structure having its ends secured to said annularreinforcing cores, and a neutral profile, lying in a radialcross-sectional plane, axially extending from bead to bead, said methodcomprising the steps of: eliminating the inflection points along theextension of said neutral profile of the reinforcing structure; causingsaid neutral profile to intersect the cross section of the zone whichencloses said annular reinforcing cores.